The spray-dried detergent powders currently sold in most European countries contain relatively large quantities of sodium tripolyphosphate which acts simultaneously as a highly efficient detergency builder and as a structurant or matrix material for carrying the organic components, notably anionic and nonionic sufactants, present in the powder. Sodium tripolyphosphate hexahydrate, under the right conditions, crystallises during detergent slurry processing as a mass of small needle-shaped crystals which on spray-drying become interspersed with small pores predominantly less than 10 .mu.m: such a pore size distribution is ideally suited to carrying mobile organic detergent components.
In recent years, it has been recognised that high levels of environmental phosphate cause eutrophication of inland waters and that phosphate-containing detergents may contribute to this. As a result various low phosphate or zero-phosphate detergency builder systems have been developed to replace sodium tripolyphosphate. One material that is cheap, readily available and has the requisite water-softening properties is sodium carbonate, and this is widely used in countries, for example, certain states of the USA, which impose a total ban on phosphates in detergents.
As a structurant or matrix material the sodium carbonate available as commerical grades of soda ash is far from satisfactory. These commerical anhydrous materials, when slurried in water at typical detergent slurry-making temperatures, crystallise as sodium carbonate monohydrate in the form of large crystals up to 100-200 .mu.m in size. As a result, the particles formed by spray-drying are interspersed with large pores of the order of 100 .mu.m in diameter. While the porosity within such particles may be adequate to absorb mobile organic components, the pores are in fact so large that such components will tend to "bleed". This will cause carton staining when the powder is stored in a cardboard carton, because the carton walls contain smaller pores than those holding the mobile components in the carbonate base, so that transfer of such components from the base to the carton is able to occur owing to capillary action.
SodiUm sulphate is also a well-known component of detergent compositions. When a slurry containing sodium carbonate and sodium sulphate is prepared, the anhydrous double salt burkeite (2Na.sub.2 SO.sub.4.Na.sub.2 CO.sub.3) can be formed to the extent that the proportions of the two salts present allow. This material, unlike sodium carbonate monohydrate, forms small crystals (about 10 .mu.m), but they are packed together in dense aggregates. The presence of Burkeite has generally been regarded as a problem, largely because of the very low porosity resulting from the dense packing.
It has now been discovered that both sodium carbonate monohydrate and Burkeite can be converted to a more desirable crystal form in the slurry by the addition of a low level of a polycarboxylate material at a particular stage in the slurry-making process. The resulting modified crystal morphology is beneficial to the uptake and retention of mobile organic components.
It is essential that the polycarboxylate crystal growth modifier be present in the slurry before crystallisation of the relevant species occurs, that is to say, it must be incorporated not later than the revelant salts. This principle can be utilised to form a simple inorganic spray-dried base, a whole detergent powder, or any intermediate product.
Crystal-growth-modified spray-dried sodium carbonate monohydrate and Burkeite in accordance with the invention contain small crystals similar to those of sodium tripolyphosphate hexahydrate, and can be shown by mercury porosimetry to be interspersed to a large extent with very small (&lt;3.5 .mu.m) pores. These powders are capable of absorbing and retaining substantial quantities of liquid nonionic surfactants and other organic detergent components as a direct result both of a decrease in crystal size and of a less dense form of crystal packing, giving particles of greater porosity than those produced in the absence of a crystal growth modifier. The modified crystal structure can be recognised by optical or electron microscopy.